Added String equation handler.

Core/src/main/java/com/songoda/core/math/Eval.java

@@ -0,0 +1,87 @@
+package com.songoda.core.math;
+
+public class Eval {
+
+    private int pos = -1, ch;
+    private final String toParse;
+    private final String warningMessage;
+
+    public Eval(String toParse, String warningMessage) {
+        this.toParse = toParse;
+        this.warningMessage = warningMessage + " ";
+    }
+
+    private void nextChar() {
+        ch = (++pos < toParse.length()) ? toParse.charAt(pos) : -1;
+    }
+
+    private boolean eat(int charToEat) {
+        while (ch == ' ') nextChar();
+        if (ch == charToEat) {
+            nextChar();
+            return true;
+        }
+        return false;
+    }
+
+    public double parse() {
+        nextChar();
+        double x = parseExpression();
+        if (pos < toParse.length()) throw new RuntimeException(warningMessage + "Unexpected: " + (char)ch);
+        return x;
+    }
+
+    // Grammar:
+    // expression = term | expression `+` term | expression `-` term
+    // term = factor | term `*` factor | term `/` factor
+    // factor = `+` factor | `-` factor | `(` expression `)`
+    //        | number | functionName factor | factor `^` factor
+
+    private double parseExpression() {
+        double x = parseTerm();
+        for (;;) {
+            if      (eat('+')) x += parseTerm(); // addition
+            else if (eat('-')) x -= parseTerm(); // subtraction
+            else return x;
+        }
+    }
+
+    private double parseTerm() {
+        double x = parseFactor();
+        for (;;) {
+            if      (eat('*')) x *= parseFactor(); // multiplication
+            else if (eat('/')) x /= parseFactor(); // division
+            else return x;
+        }
+    }
+
+    private double parseFactor() {
+        if (eat('+')) return parseFactor(); // unary plus
+        if (eat('-')) return -parseFactor(); // unary minus
+
+        double x;
+        int startPos = this.pos;
+        if (eat('(')) { // parentheses
+            x = parseExpression();
+            eat(')');
+        } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
+            while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
+            x = Double.parseDouble(toParse.substring(startPos, this.pos));
+        } else if (ch >= 'a' && ch <= 'z') { // functions
+            while (ch >= 'a' && ch <= 'z') nextChar();
+            String func = toParse.substring(startPos, this.pos);
+            x = parseFactor();
+            if (func.equals("sqrt")) x = Math.sqrt(x);
+            else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
+            else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
+            else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
+            else throw new RuntimeException(warningMessage + "Unknown function: " + func);
+        } else {
+            throw new RuntimeException(warningMessage + "Unexpected: " + (char)ch);
+        }
+
+        if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation
+
+        return x;
+    }
+}

Core/src/main/java/com/songoda/core/math/MathUtils.java

@@ -0,0 +1,17 @@
+package com.songoda.core.math;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class MathUtils {
+
+    private static final Map<String, Double> cache = new HashMap<>();
+
+    public static double eval(String toParse) {
+        return eval(toParse, "SongodaCore Eval Engine");
+    }
+
+    public static double eval(String toParse, String warningMessage) {
+        return cache.computeIfAbsent(toParse, t -> new Eval(toParse, "[" + warningMessage + "]").parse());
+    }
+}